refactor: core.sx self-declares its libc #library

core.sx owns the libc escape hatches (libc_write / libc_malloc / libc_free /
memcpy / memset, all `extern libc "..."`) but never declared the `libc`
#library constant — it free-rode on `#library` being program-global, satisfied
by other std modules (socket/fs/cli) in a full std build. So core.sx — and
list.sx, which imports it for Allocator/List — could not be imported without
assembling the whole std prelude (`extern library 'libc' is not declared`).

Declare `libc :: #library "c"` in core.sx itself. `#library` constants are
program-global and dedup, so this is harmless alongside the other declarers,
and it makes core.sx / list.sx self-contained — importable standalone. No
snapshot drift (a #library decl adds no type/global), full suite green 817/0.

library/modules/std/core.sx

@@ -4,6 +4,13 @@
 // never import this file directly — std.sx re-exports every name here.
 
 Vector :: ($N: int, $T: Type) -> Type #builtin;
+// The C library, declared HERE because core.sx owns the libc escape hatches
+// (`libc_write`/`libc_malloc`/`libc_free`/`memcpy`/`memset` below). `#library`
+// constants are program-global and dedup, so this is harmless when other std
+// modules (socket/fs/cli) also declare it — and it makes core.sx (hence
+// list.sx, which imports core for `Allocator`/`List`) self-contained, so they
+// can be imported without assembling the whole std prelude.
+libc :: #library "c";
 // `out` writes a string straight to fd 1 via libc `write` — a plain sx function,
 // NOT a compiler builtin. At comptime it runs through the evaluator's host-FFI
 // escape (the VM's dlsym path / the interp's extern call); at runtime it's an